A process used for converting gas to liquid petroleum can be accomplished using a Fischer-Tropsch catalyst. Since the invention of the original process by Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s, many refinements and adjustments have been made to this process. The term “Fischer-Tropsch” now applies to a wide variety of similar processes (Fischer-Tropsch synthesis or Fischer-Tropsch chemistry). Fischer and Tropsch filed a number of patents, e.g., U.S. Pat. No. 1,746,464, related to this process.
The Fischer-Tropsch process involves a series of chemical reactions that lead to a variety of hydrocarbons (CnH(2n+2)). Useful reactions give alkanes:(2n+1)H2+nCO→CnH(2n+2)+nH2O  (1)
where the term “n” represents a positive integer. The formation of methane (n=1) is generally unwanted. Most of the alkanes produced tend to be straight-chain hydrocarbons, suitable for use as a diesel fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons.
Several reactions are employed to adjust the H2/CO ratio. Most important is the water gas shift reaction, which provides a source of hydrogen at the expense of carbon monoxide:H2O+CO→H2+CO2  (2)
For Fischer-Tropsch plants that use methane as the feedstock, another important reaction is steam reforming, which converts the methane into CO and H2:H2O+CH4→CO+3H2  (3)
The conversion of CO to alkanes involves hydrogenation of CO, the hydrogenolysis (cleavage with H2) of C—O bonds, and the formation of C—C bonds. Such reactions are assumed to proceed via initial formation of surface-bound metal carbonyls. The CO ligand is speculated to undergo dissociation, possibly into oxide and carbide ligands. Other potential intermediates are various C-1 fragments including formyl (CHO), hydroxycarbene (HCOH), hydroxymethyl (CH2OH), methyl (CH3), methylene (CH2), methylidyne (CH), and hydroxymethylidyne (COH). Furthermore, and critical to the production of liquid fuels, are reactions that form C—C bonds, such as migratory insertion. Many related stoichiometric reactions have been simulated on discrete metal clusters, but homogeneous Fischer-Tropsch catalysts are poorly developed and of little commercial importance.
Generally, the Fischer-Tropsch process is operated in the temperature range of 150-300° C. (302-572° F.). Higher temperatures usually lead to faster reactions and higher conversion rates, but also tend to favor methane production. For this reason, the temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favors formation of long-chained alkanes, both of which are desirable. Typical pressures range from one to twenty atmospheres. High pressure may reduce the reaction temperature which would make the Fischer-Tropsch process compatible with most oilfield operations.
A variety of synthesis-gas compositions can be used. For cobalt-based catalysts the optimal H2:CO ratio is around 1.8-2.1. Iron-based catalysts promote the water-gas-shift reaction. Accordingly, iron-based catalysts can tolerate lower ratios of H2:CO. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H2:CO ratios (<1).
In general the product distribution of hydrocarbons formed during the Fischer-Tropsch process follows an Anderson-Schulz-Flory distribution, which can be expressed as:Wn/n=(1−α)2αn−1  (4)
Where Wn is the weight fraction of hydrocarbon molecules containing n carbon atoms. The term “α” represents the chain growth probability or the probability that a molecule will continue reacting to form a longer chain. In general, α is largely determined by the catalyst and the specific process conditions.
Examination of equation (4) reveals that methane will always be the largest single product so long as α is less than 0.5. However, by increasing value of α to about one, the total amount of methane formed can be minimized compared to the sum of all of the various long-chained products. Increasing α increases the formation of long-chained hydrocarbons. The very long-chained hydrocarbons are waxes, which are solid at room temperature. Therefore, for production of liquid transportation fuels it may be necessary to crack some of the Fischer-Tropsch products. In order to avoid this, some researchers have proposed using zeolites or other catalyst substrates with fixed sized pores that can restrict the formation of hydrocarbons longer than some characteristic size (usually n<10). This way they can drive the reaction so as to minimize methane formation without producing lots of long-chained hydrocarbons. Such efforts have met with only limited success.
A variety of catalysts can be used for the Fischer-Tropsch process, but the most common are the transition metals cobalt, iron, and ruthenium. Cobalt, nickel, iron, molybdenum, tungsten, thorium, ruthenium, rhenium and platinum are known to be catalytically active, either alone or in combination, in the conversion of synthesis gas into hydrocarbons and oxygenated derivatives thereof. Of the aforesaid metals, cobalt, nickel and iron have been studied most extensively. Nickel tends to favor methane formation (“methanation”). Generally, the metals are used in combination with a support material, of which the most common are alumina, silica and carbon.
Cobalt-based catalysts are highly active, although iron may be more suitable for low-hydrogen-content synthesis gases such as those derived from coal due to its promotion of the water-gas-shift reaction. In addition to the active metal, the catalysts typically contain a number of “promoters,” including potassium and copper. Group 1 alkali metals, including potassium, are a poison for cobalt catalysts but are promoters for iron catalysts. Catalysts are supported on high-surface-area binders/supports such as silica, alumina, or zeolites. Cobalt catalysts are more active for Fischer-Tropsch synthesis when the feedstock is natural gas. Natural gas has a high hydrogen to carbon ratio, so the water-gas-shift is not needed for cobalt catalysts. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass.
Unlike the other metals used for this process (Co, Ni, Ru), which remain in the metallic state during synthesis, iron catalysts tend to form a number of phases, including various oxides and carbides during the reaction. Control of these phase transformations can be important in maintaining catalytic activity and preventing breakdown of the catalyst particles.
Fischer-Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. The sensitivity of the catalyst to sulfur is greater for cobalt-based catalysts than for their iron counterparts.
Promoters also have an important influence on activity. Alkali metal oxides and copper are common promoters, but the formulation depends on the primary metal. Alkali oxides on cobalt catalysts generally cause activity to drop severely even with very low alkali loadings. C5+ and CO2 selectivity increase while methane and C2-C4 selectivity decrease. In addition, the olefin to paraffin ratio increases.
The use of cobalt as a catalytically active metal in combination with a support has been described in, for example, EP-A-127220, EP-A-142887, GB-A-2146350, GB-A-2130113 and GB-A-2125062. EP-A-127220, for example discloses the use of a catalyst comprising (i) 3-60 pbw cobalt, (ii) 0.1-100 pbw zirconium, titanium, ruthenium or chromium, per 100 pbw silica, alumina or silica-alumina, (iii) the catalyst having been prepared by kneading and/or impregnation.
EP 261870 describes a composition for use after reductive activation as a catalyst for the conversion of synthesis gas to hydrocarbons comprising as essential components (i) cobalt either as the elemental metal, oxide or a compound thermally decomposable to the elemental metal or oxide and (ii) zinc in the form of the oxide or a compound thermally decomposable to the oxide. The resultant catalysts, in contrast to many prior art cobalt-containing catalysts, are more selective to hydrocarbons in the C5-C120 range and can be very selective to a waxy hydrocarbon product. These catalysts may also contain in elemental form or oxide form one or more of the following metals as promoters: chromium, nickel, iron, molybdenum, tungsten, zirconium, gallium, thorium, lanthanum, cerium, ruthenium, rhenium, palladium or platinum suitably in amount up to 15% w/w. Exemplified compositions included chromium, zirconium, gallium and ruthenium as promoters.
U.S. Pat. No. 4,039,302 describes a catalyst containing cobalt oxide and zinc oxide for use in the synthesis of C1-C3 aliphatic hydrocarbons
U.S. Pat. No. 4,826,800 describes a process for preparing a catalyst comprising cobalt and zinc oxide for use after reductive activation as a catalyst in the conversion of synthesis gas to hydrocarbons. The catalyst is prepared by mixing a solution, of a soluble zinc salt and a soluble cobalt salt with a precipitant such as ammonium hydroxide or ammonium carbonate and recovering the precipitate. The ratio of carbonate to metal is high in the described method, which has been found detrimental to the strength of the catalyst.
U.S. Pat. No. 5,345,005 relates to a Cu—Zn catalyst on alumina for the preparation of alcohols by hydrogenation of e.g. a ketone. In a comparative example, the preparation of a Cu—Zn—Co catalyst on alumina is described, wherein use is made of soda ash. However, the use of soda ash is found to be potentially detrimental to the strength of the catalyst. The particle size distribution range within which 90% of the volume of the Cu—Zn—Co catalyst described in U.S. Pat. No. 5,345,005 lies, is not specified. It is however expected that the use of soda ash in the preparation of the catalyst leads to a broadening in the particle size distribution.
U.S. Pat. No. 5,945,458 and U.S. Pat. No. 5,811,365 describe a Fischer-Tropsch process in the presence of a catalyst composition of a group VIII metal, e.g. cobalt, on a zinc oxide support. Such a catalyst is made by first preparing the support by adding a solution of zinc salt and other constituents to an alkaline bicarbonate solution. Next, the precipitate is separated from the bicarbonate solution by filtration to form a filter cake, which can thereafter be dried, calcined and loaded with the group VIII metal. The catalyst material is then formed into tablets, which tablets are crushed to form particles with a size of 250-500 μm, that can be used in a Fischer-Tropsch process. Additional post-treatments such as crushing, are required in order to obtain a catalyst powder with good strength properties. However, the obtained average particle size; as indicated above, is still relatively large. Moreover, crushing results in a broad particle size distribution and catalysts with such a large particle size and a broad particle size distribution tend to be less suitable for processes involving a bubble column, a slurry phase reactor or a loop reactor.
WO-A-01/38269 describes a three-phase system for carrying out a Fischer-Tropsch process wherein a catalyst suspension in a liquid medium is mixed with, gaseous reactants in a high shear mixing zone, after which the mixture is discharged in a post mixing zone. Thus mass transfer is said to be enhanced. As suitable catalysts inter alia cobalt catalysts on an inorganic support, such as zinc oxide are mentioned. The surface area of the support used for the preparation of these known catalysts is less than 100 g/m2. These prior art cobalt based catalysts can be prepared by depositing cobalt on a suitable support, such as a zinc oxide support, by impregnation methodology. Other conventional preparation methods include precipitation routes, which typically involve crushing of a hard filter cake of catalyst material, resulting from the catalyst preparation process, into small particles.
WO 03/090925 describes a Fischer-Tropsch catalyst comprising particles of a cobalt and zinc co-precipitate having specific volume average particle size and particle size distributions. The catalysts essentially consist of cobalt and zinc oxide but may also contain other components commonly employed in Fischer-Tropsch catalysts such as ruthenium, hafnium, platinum, zirconium, palladium, rhenium, cerium, lanthanum, or a combination thereof. When present such promoters are typically used in a cobalt to promoter atomic ratio of up to 10:1.
EP 221598 describes supported catalysts comprising a metal component of iron, nickel or cobalt promoted by zirconium and in addition a noble metal from Group VIII of the Periodic Table. The catalysts are suitable for the preparation of hydrocarbons from carbon monoxide and hydrogen. Preferred noble metals include platinum or palladium and the catalysts are most suitably supported on silica or alumina.
Fischer-Tropsch plants associated with coal or related solid feedstocks (sources of carbon) must first convert the solid fuel into gaseous reactants, i.e. CO, H2, and alkanes. This conversion is called gasification. Synthesis gas obtained from coal gasification tends to have a H2/CO ratio of about 0.7 compared to the ideal ratio of about 2. This ratio is adjusted via the water-gas shift reaction. Coal-based Fischer-Tropsch plants can produce varying amounts of CO2, depending upon the energy source of the gasification process. However, most coal-based plants rely on the feed coal to supply all the energy requirements of the Fischer-Tropsch process.
In summary, the Fischer-Tropsch process is performed by forcing hydrogen and carbon monoxide over a catalyst. The gases recombine into water and a petroleum product which can range between diesel and paraffin. Heat and water are also generated in this process. Currently, there is a need to improve the efficiency of this process. The present disclosure presents a method and an apparatus that improves the efficiency of catalytic processes.